Mosquitoes Have the Edge in Singapore’s Dengue War

Singapore and its Southeast Asian neighbors are in the midst of a new epidemic of dengue that is on course to claim more victims regionally than the last epidemic, in 2005.

WAYNE ARNOLD

SINGAPORE, June 26 — Under the sink, behind the cleaning detergents, Thurainadan Govindarajoo shined his flashlight into the shadows, searching for telltale signs of the enemy.

“People only think of the obvious places,” he said. “We’re looking for what I call the hidden habitats.” Under leaking sinks, in disused toilets, beneath potted plants: wherever a few drops of water can linger, mosquitoes can breed.

Mr. Govindarajoo is one of roughly 500 inspectors from Singapore’s National Environment Agency specially trained to conduct house-to-house search-and-destroy missions against Aedes mosquitoes, which transmit the potentially deadly dengue virus. Despite their best efforts, though, the mosquitoes appear to be winning, abetted by the boom in international travel, global warming and their own adaptability.

Singapore and its Southeast Asian neighbors are in the midst of a new epidemic of dengue (pronounced DEN-gay) that is already on course to claim more victims regionally than the last epidemic, in 2005.

Thailand has already had more than 11,000 reported cases so far this year, with 14 deaths, while 48 people have died among Malaysia’s more than 20,000 dengue cases. Sprawling Indonesia, with more than 68,000 reported cases, has had 748 deaths. And while Singapore’s two dengue-related deaths give it the lowest fatality rate in the region, its nearly 3,000 cases make its infection rate second only to Malaysia’s.

Dengue is a relative of yellow fever, hepatitis C and the West Nile virus. It infects an estimated 50 million people a year, and there remains no vaccine or treatment. In acute cases, it causes high fever and debilitating lethargy, accompanied by joint pain so intense that the disease was called “breakbone fever” when it was first diagnosed more than 300 years ago. About 1 percent of these more serious cases develop hemorrhagic fever or shock, with gastrointestinal bleeding and, in rare cases, brain hemorrhages and death.

Although there are incidences of dengue around the year in tropical climates, cases tend to spike during periods of high rainfall and high temperatures.

If there is a silver lining to Singapore’s dengue problem, it is that it has turned the island state into a global center for knowledge about the disease. As Singapore has promoted itself as a hub for biotechnology research, dengue has quickly become a common cause. In 2004, for example, the pharmaceutical company Novartis opened an Institute for Tropical Disease here, with a specific focus on developing a treatment for dengue.

Nonetheless, results have been slow in coming. “There’s no quick fix here,” said Paul Herrling, head of corporate research at Novartis.

Dengue is an enigmatic virus, difficult to diagnose and impossible to quarantine. Ninety percent of those infected with dengue develop only mild flulike symptoms, if they feel anything at all, making them unwitting reservoirs for the virus.

Even when symptoms appear, they do so days after the patient has become infectious. And after the onset of dengue’s characteristic fever that varies widely in temperature, antibodies do not appear in significant levels for days, meaning doctors cannot use conventional blood tests to detect the virus until the worst is already over.

Creating a vaccine against dengue might be a simple matter if it were not for another quirk of the virus. Dengue has four known strains, and while infection with one strain appears to provide lifelong immunity against that strain and one of the others, it seems to make a person more likely to hemorrhage if infected with one of the other two strains. Any vaccine, therefore, would have to work simultaneously against all four strains.

Because dengue was long confined to the tropics, it remains a little-understood disease. Experts still do not know precisely how the virus affects the liver or why it causes the level of blood-clotting platelets in the bloodstream to decline.

The virus is spreading fast. Global warming is extending the Aedes mosquitoes’ habitat so that dengue has now marched north from Latin America to the southern United States. Commercial aviation is also carrying infected individuals — and Aedes mosquitoes — to new areas.

If the virus cannot be blocked or the people carrying it detected and quarantined, the only logical alternative is to thwart the mosquito that transmits it. Singapore’s mosquito control program is widely recognized as the world’s most rigorous. There are widespread public awareness campaigns. Posters around the island now picture a black-and-white-striped, blood-engorged Aedes with a warning: “If they breed, you will bleed.”

Doctors here are required by law to report dengue cases to the Ministry of Health, which sends the information daily to the National Environment Agency. At the agency, the cases are plotted on an islandwide map in a dedicated dengue situation room. Any area where two cases occur within 14 days of each other in a 150-meter radius is designated a hot spot, and inspectors like Mr. Govindarajoo are dispatched to scour the area for mosquito breeding sites.

The inspectors called to the Indonesian maid at a house in Lengkong, a neighborhood where seven dengue cases had erupted in recent weeks: “Hello. Good morning. Selamat pagi. Checking for mosquitoes.”

The inspectors fanned out, some climbing ladders to check gutters, while others searched the garden for standing water in potted plants or between the leaves of palm trees.

Aedes aegypti, the most prolific transmitter of dengue, has become ideally suited to the rapidly growing tropical urban environment. Unlike malaria-transmitting mosquitoes that stick to rural areas and swampy waters, it prefers fresh, clean water. It breeds largely indoors, needing only tiny pools of water to lay its eggs. Christina Liew, a medical entomologist at the agency, said Aedes mosquitoes are not as fussy about where they will lay eggs as was once believed. In the absence of clean water, Ms. Liew said, females will lay eggs in polluted water.

“They have learned to adapt to urban situations,” she said.

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